Claus Ibsen created CAMEL-26298:
-----------------------------------
Summary: camel run --mcp-stdio fails at startup: No
McpJsonMapperSupplier available
Key: CAMEL-26298
URL: https://issues.apache.org/jira/browse/CAMEL-26298
Project: Camel
Issue Type: Bug
Components: camel-mcp-server, camel-jbang
Affects Versions: 4.23.0
Reporter: Claus Ibsen
Fix For: 4.24.0
{{camel run --mcp-stdio}} does not start, in Camel 4.23.0 and on main
(4.24.0-SNAPSHOT).
*How to reproduce*
{{tools.yaml}}:
{code:yaml}
- route:
from:
uri: ai-tool:hello
parameters:
tags: demo
description: Say hello
steps:
- setBody:
expression:
constant:
expression: Hello
{code}
{code}
camel run tools.yaml --mcp-stdio --mcp-tags=demo
camel run tools.yaml --camel-version=4.23.0 --mcp-stdio --mcp-tags=demo
{code}
Both stop at startup with:
{code}
java.util.ServiceConfigurationError: No McpJsonMapperSupplier available for
creating McpJsonMapper
at
io.modelcontextprotocol.util.McpServiceLoader.getDefault(McpServiceLoader.java:59)
at
io.modelcontextprotocol.json.McpJsonDefaults.getMapper(McpJsonDefaults.java:58)
at
org.apache.camel.component.mcp.server.stdio.StdioMcpServerEngine.initialize(StdioMcpServerEngine.java:85)
at
org.apache.camel.component.mcp.server.McpServerBridge.doInit(McpServerBridge.java:135)
...
at
org.apache.camel.main.BaseMainSupport.setupMcpServer(BaseMainSupport.java:2326)
{code}
camel-mcp-server has {{mcp-json-jackson2}} as a normal dependency, so the jar
is on the classpath. Most likely the MCP SDK's service lookup does not see it
in the class loader that Camel JBang uses for downloaded dependencies (not
verified). The stdio tests in camel-mcp-server run on a normal classpath, so
they do not catch this.
The 4.23 release notes and the mcp-server documentation show {{camel run
tools.yaml --mcp-stdio}}, including a Claude Desktop configuration, so users
will try it.
*Second problem: route output goes into the MCP stream*
In stdio mode the MCP protocol is written to {{System.out}}, and nothing
redirects {{System.out}} ({{StdioMcpServerEngine.setTransportStreams}} is never
called). Logging goes to stderr, which is fine. But a route that writes to
{{stream:out}}, a {{System.out.println}} in a bean, or a library banner writes
into the MCP stream and breaks the client. This could not be tested yet,
because of the startup error.
Suggested fix: in stdio mode, give the MCP transport the original
{{System.out}} ({{setTransportStreams}}), and point {{System.out}} at stderr.
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